1. Field of the Invention
The invention relates to a method of dispensing a fluidizable solid from a pressure vessel having a weighing device and closable supply and discharge, wherein of three gas flows which are supplied to the solid and are at various pressure values P.sub.1, P.sub.2, P.sub.3, the gas flow at medium pressure P.sub.2 is conducted into the vessel above the solid and the gas flow at relatively high pressure P.sub.1 is conducted into the lower vessel region for fluidizing the solid, and the gas flow or relatively low pressure P.sub.3 is conducted into the discharge connected thereto, the latter opening into a chamber at very low pressure P.sub.4, and the quantity of solid taken from the vessel being weighed and also to an apparatus for carrying out the method.
2. Discussion of the Prior Art
Some method steps and the apparatus required for these in the method described hereinbefore are known from publications: German Auslegeschrift No. 1,140,863, German Pat. No. 1,147,756 and German Auslegeschrift No. 1,148,938.
Further developments on the basis of these have led to a known procedure and an apparatus such as is illustrated in FIG. 1 and FIG. 3 of the accompanying drawings and will be described in detail hereinafter.
In FIG. 1, the apparatus includes a pressure vessel 1 having a closable charging aperture 2 through which solid 3 is introduced and a closable discharge 4, 19. Conduits 5, 7 and 8 are connected to gas pressure sources and a pressure reducing valve 12 is provided. The gas pressure conduit 7 leads into a chamber 9 formed between the vessel bottom end 10 and a second bottom end 11 which is arranged above bottom 10 and which is gas-pervious. A weighing device 14 for the solid 3 is connected to the vessel 1. There is a dispensing or consumption chamber 17 in which a very low pressure P.sub.4 prevails.
The known apparatus illustrated operates as follows:
With the discharge 4 closed, the pressure vessel 1 is filled with a solid 3 in powder form. The charging aperture 2 is closed and the gas pressure sources 5, 7, 8 are connected, the gas pressure source 5 being used for regulating the top pressure P.sub.2 the gas pressure source 7 for fluidizing the solid at pressure P.sub.1 and the gas pressure source 8 for supplying the "conveying gas" at pressure P.sub.3. The gas supplied to the chamber 17 and providing pressure P.sub.4 is the gas from the pressure source through 7 and the gas from the pressure source through 8. The prerequisite for conveying the fluidized solid is that the pressure relationship should be P.sub.1 &gt;P.sub.2 &gt;P.sub.3 &gt;P.sub.4, P.sub.1 being the pressure of the second gas pressure source through 7, P.sub.2 the pressure of the first gas pressure source through 5, P.sub.3 the pressure of the third gas pressure source through 8 and P.sub.4 the pressure in the chamber 17. By means of the weighing apparatus 14 the quantity of solid to be dispensed from the vessel is determined. The known method is based on the relationship shown in FIG. 3, according to which the conveyed quantity of solid .[.O.]. .Iadd.Q .Iaddend.(e.g., in kg/min) depends on the pressure difference P.sub.2 -P.sub.3 : Q=const. (P.sub.2 -P.sub.3).
If the weighing apparatus 14 indicates for example that insufficient solid 3 is being conveyed, the top pressure P.sub.2 is increased above the solid 3 in the vessel 1. As FIG. 3 shows more particularly, with rising top pressure P.sub.2, plotted on the abscissa, the conveyed quantity of solid Q plotted on the ordinate is increased. Owing to the increased quantity of solid Q per volume unit conveyed, however, the pressure P.sub.3 of the conveying gas is also increased, but to a lesser extent than the top pressure P.sub.2. Consequently, with increasing top pressure P.sub.2 the difference between top pressure P.sub.2 and conveying pressure P.sub.3 increases, this being therefore in direct relationship to the throughflow of the quantity of solid. This relationship is usually used for controlling the throughflow of the solid concerned.
To reduce the conveyed solid quantity Q the top pressure P.sub.2 is reduced in accordance with the procedure described. A device for discharging gas can be provided additionally at the upper end of the vessel.
The control described does not influence the gas flow at relatively high pressure and the gas flow at relatively low pressure P.sub.2, P.sub.3, which exclusively determine the gas quantity and therefore the relation of gas to solid. The pressure reducing valve 12 which is associated with the third gas pressure through 8 and which operates like a proportional controller without a servo force, also discharges varying gas quantities if the pressure P.sub.4 in the chamber 17 of lowest pressure fluctuates. This would be the case for example when moving an immersion lance into a molten metal bath. The pressure reducing valve 12 provided has the result, when this pressure change occurs, of producing the conveyance of a smaller quantity of gas. On the other hand the pressure reducing valve 12 when the pressure P.sub.4 falls operates in such a manner that an increased quantity of gas is conveyed, in order thus to re-establish constant pressure conditions.
This has the result that for example when moving an immersion lance into and out of a torpedo ladle, with high throughflow quantities, liquid metal is thrown out of the ladle. To avoid this it is necessary to limit the degree to which the torpedo ladle is filled, thus reducing the working capacity.
In the constructional form described in FIG. 1 there also occurred when there was a considerable pressure drop in the chamber 17 of the very low pressure P.sub.4, blockages in the region of the chamber 9 since the opening of the pressure reducing valve 12 which results caused a corresponding pressure drop of P.sub.1 in the chamber 9 and the pressure P.sub.1 became too small in relation to the pressure P.sub.2. The results of this are inadequate fluidization of the solid and non-uniform dispensing.